Electric power steering system

ABSTRACT

An electric power steering system includes a steering wheel, a rack-and-pinion type steering gear box connected to the steering wheel, a steering torque sensor, a pinion shaft, and a steering actuator mounted on the pinion shaft to assist a steering operation conducted by a driver. An overload preventing control is conducted so that a sum of a current conversion value of a steering torque detected by the steering torque sensor and an assist current calculated based on the steering torque becomes equal to a limit value. When the steering gear box has reached a stroke end, the assist current is prevented from excessively increasing, thereby preventing a reduction in durability of the steering gear box or a motor of the steering actuator.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention claims priority under 35 USC 119 based on Japanesepatent application No. 2006-325628, filed on Dec. 1, 2006. The entiretyof the subject matter of this priority document is incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric power steering system thatprevents an assist current from excessively increasing and detrimentallyaffecting the durability of a gear box and/or the motor of a steeringactuator.

2. Description of the Related Art

Japanese Patent Application Laid-open No. 2002-234456 (JP '456)discloses an electric power steering system having a steering devicewhich includes a rack-and-pinion type steering gear box, wherein asteering torque is input from a steering actuator to the steering deviceto assist a steering operation conducted by a driver. In the steeringdevice disclosed by JP '456, overheating of a motor of the steeringactuator or a control system due to an overload is prevented byproviding an upper limit value on an indicator current supplied to themotor of the steering actuator.

Limiting of the upper limit value of the indicator current supplied tothe motor is initiated when a continuous service time of the motorexceeds a predetermined amount. The upper limit value is first graduallydecreased from a predetermined maximum value to a minimum value at apredetermined gradually decreasing rate. The upper limit value of thesupplied indicator current is then increased to the maximum value at apredetermined gradually increasing rate. The conditions for limiting theupper limit value of the supplied indicator current are less severe whena temperature sensor detects a decrease in the atmospheric temperature.

In the steering device disclosed by JP '456, when a rack bar of thesteering gear box cannot be further moved after reaching a lateralstroke end, the steering torque detected by the steering torque sensoris steeply increased. Therefore, an assist current supplied to the motorof the steering actuator is also steeply increased. In order toaccommodate such a steep increase in the assist current, theassist-current limiting technique disclosed by JP '456 is insufficient.Therefore, there is a need in the industry for a more reliableassist-current limiting technique.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the abovecircumstances and has an aspect to reliably prevent overloading eitherone of a steering gear box or a steering actuator of an electric powersteering system.

In order to achieve the above aspect, according to a first feature ofthe present invention, there is provided an electric power steeringsystem including: a steering wheel; a rack-and-pinion type steering gearbox connected to the steering wheel; a steering torque sensor fordetecting a steering torque input into the steering wheel; and asteering actuator driven by an assist current. The assist current iscalculated based on the detected steering torque in order to assist asteering operation conducted by a driver. An overload preventing controlfor limiting the driving of the steering actuator is conducted so that asum of the assist current and a current conversion value of the steeringtorque detected by the steering torque sensor becomes a predeterminedthreshold value.

Based on the structural arrangement of the first feature, when thesteering operation conducted by the driver is assisted by the steeringactuator of the electric power steering system, the overload preventingcontrol is conducted so that the sum of the conversion current value ofthe steering torque detected by the steering torque sensor and theassist current calculated based on the steering torque becomes equal tothe predetermined threshold value. Therefore, it is possible to suppressthe assist current from excessively increasing when the steering gearbox has reached the stroke end, thereby preventing the durability of thesteering gear box or the steering actuator from being reduced.

According to a second feature of the present invention, in addition tothe first feature, when the steering torque detected by the steeringtorque sensor is equal to or larger than a first threshold value, orwhen the assist current is equal to or larger than a second thresholdvalue, the overload preventing control is conducted.

Based on the structural arrangement of the second feature, when thesteering torque detected by the steering torque sensor is equal to orlarger than the first threshold value, or when the assist currentcalculated based on the steering torque is equal to or larger than thesecond threshold value, the overload preventing control is conducted.Therefore, when an overload of the steering gear box or the steeringactuator is liable to be generated in a transmitting system of steeringtorque, the overload preventing control is conducted.

According to a third feature of the present invention, in addition tothe first feature, the system further includes a stroke end sensor fordetecting a stroke end of the steering gear box. Therefore, when thestroke end is detected and the assist current is smaller than thethreshold value, the overload preventing control is conducted using athreshold value that is less or lower than the predetermined thresholdvalue.

Based on the structural arrangement of the third feature, when thestroke end sensor detects the stroke end of the steering gear box in astate in which the assist current is smaller than the predeterminedthreshold value, the overload preventing control is conducted using thelower threshold value. Therefore, even if the assist current is smalldue to a small friction coefficient of a road surface when the steeringgear box reaches the stroke end, the overload preventing control isstarted. Further, the threshold value on the overload preventing controlis decreased to further reliably prevent an overload of either one ofthe steering gear box or the steering actuator.

According to a fourth feature of the present invention, in addition tothe first feature, the system further includes a stroke end sensor fordetecting a stroke end of the steering gear box. Therefore, when thestroke end is detected and the steering torque is smaller than thepredetermined threshold value, the overload preventing control isconducted using a threshold value that is less or lower than thepredetermined threshold value.

Based on the structural arrangement of the fourth feature, when thestroke end sensor detects the stroke end of the steering gear box in astate in which the steering torque is smaller than the predeterminedthreshold value, the overload preventing control is conducted using thelower threshold value. Therefore, even if the steering torque is smalldue to a small friction coefficient of a road surface when the steeringgear box reaches the stroke end, the overload preventing control isstarted. Further, the threshold value on the overload preventing controlis decreased to further reliably prevent an overload of either one ofthe steering gear box or the steering actuator.

According to a fifth feature of the present invention, in addition tothe third or fourth feature, the stroke end sensor detects the strokeend based on rigidity of the steering gear box.

Based on the structural arrangement of the fifth feature, the stroke endsensor detects the stroke end based on the rigidity of the steering gearbox. Therefore, the stroke end is detected without needing a positionsensor for detecting the position of a rack bar.

According to a sixth feature of the present invention, in addition toany of the first-to-fifth features, the steering torque sensor is amagnetostriction torque sensor.

Based on the structural arrangement of the fifth feature, themagnetostriction steering torque sensor is used as a steering torquesensor. Therefore, it is possible not only to increase the rigidity ofthe steering torque transmitting system but also to increase the upperlimit value of the detectable steering torque as compared with a caseusing a steering torque sensor having a torsion bar.

The above and other aspects, features and advantages of the inventionwill become apparent from preferred embodiments taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an electric power steering systemaccording to one embodiment of the present invention;

FIG. 2 is an enlarged view of a steering torque sensor;

FIG. 3 is a cross-sectional view of the steering torque sensor takenalong line 3-3 in FIG. 2;

FIG. 4 is an exploded view of the steering torque sensor;

FIGS. 5A and 5B are diagrams explaining the operation of the steeringtorque sensor;

FIG. 6 is a graph showing characteristics in changing of a torquedetection signal with respect to a steering torque;

FIG. 7 is a block diagram of a control system of a steering actuator;

FIG. 8 is a flowchart explaining the control of a steering actuator;

FIG. 9 is a schematic diagram used for explaining a stroke end sensorunit according to a second embodiment of the present invention;

FIG. 10 is a flowchart explaining the control of a steering actuator;

FIG. 11 is a schematic diagram used for explaining a stroke end sensorunit according to a third embodiment of the present invention; and

FIG. 12 is a schematic diagram used for explaining a magnetostrictionsteering torque sensor according to a fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electric power steering system in an automobile,according to an embodiment of the present invention, includes an uppersteering shaft 12 integrally rotated with a steering wheel 11; anintermediate steering shaft 14 connected to the upper steering shaft 12through an upper universal joint 13; a rack-and-pinion type steeringgear box 17 connected to the intermediate steering shaft 14 through alower universal joint 15 and a lower steering shaft 16; and a steeringactuator 18 mounted in the steering gear box 17.

The steering gear box 17 includes a housing 26, a rack bar 20 having arack 19 formed thereon; and a pinion shaft 22 having a pinion 21operationally meshing with the rack 19. The housing 26 laterally andslidably supports the rack bar 20, supports the pinion shaft 22 at alocation sandwiching the pinion 21 through a pair of ball bearings 23and 24, and supports an upper portion of the lower steering shaft 16through a ball bearing 25. Left and right opposite ends of the rack bar20 are connected to left and right wheels W, W through left and rightball joints 27, 27 and left and right tie rods 28, 28.

The steering actuator 18 includes a brushless motor 29, a worm 31provided on an output shaft 30 of the motor 29, and worm wheel 32mounted on the pinion shaft 22 and meshed with the worm 31.

The structure of a torsion-bar type steering torque sensor 33 mountedbetween a lower end of the lower steering shaft 16 and an upper end ofthe pinion shaft 22 will be described below with reference to FIGS. 2 to4.

The steering torque sensor 33 includes a pair of first guide portions 16a, 16 a formed by bifurcating a lower end of the lower steering shaft16; a pair of notches 22 a, 22 a formed by cutting off a portion of anupper end of the pinion shaft 22; and a pair of second guide portions 22b, 22 b sandwiched between the notches 22 a, 22 a. The first guideportions 16 a, 16 a are fitted into the notches 22 a, 22 a, wherein thefirst guide portions 16 a, 16 a and the second guide portions 22 b, 22 bform a circular guide surface with four clearances a (see FIG. 3)provided therebetween. A bottom of a bore 22 c formed at the center ofthe pinion shaft 22 and a ceiling surface 16 b of the lower steeringshaft 16 are connected to each other by a torsion bar 34.

A cylindrical collar 35 made of a magnetic material is movable in avertical direction and is fitted on the above-described circular guidesurface. The collar 35 is formed with a pair of first elongated bores 35a, 35 a extending in an axial direction and a pair of second elongatedbores 35 b, 35 b extending to be inclined with respect to the axialdirection. A pair of guide pins 36, 36 embedded in the first guideportions 16 a, 16 a is inserted in the pair of first elongated bores 35a, 35 a. A pair of second guide pins 37, 37 embedded in the second guideportions 22 b, 22 b is inserted in the pair of second elongated bores 35b, 35 b.

A first coil 38A and a second coil 38B are vertically disposed tosurround the collar 35 and are connected to a differential amplifyingcircuit 39 (see FIG. 1).

Thus, when a driver inputs a steering torque to the steering wheel 11,the steering torque is transmitted through the upper steering shaft 12,the upper universal joint 13, the intermediate steering shaft 14 and thelower universal joint 15 to the lower steering shaft 16, wherein thetorsion bar 34 connecting the lower steering shaft 16 and the pinionshaft 22 to each other is torsionally deformed corresponding to thesteering torque. When the lower steering shaft 16 is rotated relative tothe pinion shaft 22 by the torsional deformation of the torsion bar 34,the collar 35 is rotated along with the lower steering shaft 16. Then,the collar 35 is rotated relative to the pinion shaft 22. Hence, thecollar 35 is moved upward or downward depending on the direction of therelative rotation (see FIG. 5). At this time, the vertical movement ofthe collar 35 is permitted by the movement of the first elongated bores35 a, 35 a about the first guide pins 36, 36.

As shown in FIG. 6, when the collar 35 is vertically moved correspondingto the steering torque, changes in magnetic characteristics of the firstcoil 38A and the second coil 38B are detected as changes in first andsecond output voltages VT1 and VT2. The differential amplifying circuit39 multiplies a difference between the first and second output voltagesVT1 and VT2 by a gain k to obtain a third output voltage VT3 (torquedetection signal). The first output voltage VT1 increases with anincrease in steering torque, and the second output voltage VT2 decreaseswith the increase in steering torque. Hence, the third output voltageVT3 increases with the increase in steering torque. When the steeringtorque is 0, the third output voltage VT3 is biased to become apredetermined bias voltage Vb (for example, 2.5 V).

VT3=k×(VT1−VT2)+Vb

When the third output voltage VT3 is calculated in this manner, apredetermined steering torque T is located in a map stored in a computer(not shown) based on the third output voltage VT3 and a vehicle speed.

As shown in FIGS. 7 and 8, at Step S1, a steering torque T is detectedby the steering torque sensor 33. In Step S2, an assist current A to besupplied to the motor 29 of the steering actuator 18 is searched in anassist current map 41 based on the detected steering torque T. If theassist current A is determined to be smaller than a second thresholdvalue A1 and the steering torque T is determined to be smaller than afirst threshold value T1 in Step S3, the operation of the motor 29 iscontrolled based on the assist current A searched from the map in StepS4 (usual electric power steering control). If the assist current A isequal to or larger than the second threshold value A1 or if the steeringtorque T is equal to or larger than the first threshold value T1 in StepS3, the operation of the motor 29 is controlled at Step S5 based on aspecial overload-suppressing assist current A calculated from the assistcurrent A searched from the map.

When the assist current A is calculated in Step S4 or Step S5, the motor29 of the steering actuator 18 is subjected to a PID feedback control.Specifically, an actual current I in the motor 29 is detected by acurrent sensor 42; the actual current I is subtracted from the assistcurrent A by a subtracting means 43 to provide a difference A−I; and aPID controller 44 controls the operation of the motor 29 based on thedifference A−I so as to converge the difference of A−I to 0.

In Step S3 of the flowchart shown in FIG. 8, the situation where theassist current A is equal to or larger than the second threshold valueA1 or where the steering torque T is equal to or larger than the firstthreshold value T1 is presumed, to be a situation where the rack bar 20of the steering gear box 18 has been moved to a lateral limit position(stroke end) to reach a state in which the steering gear box 18 cannotbe moved further. In this case, in trying to increase the turning angle,the driver tends to forcefully operate the steering wheel 11, and thesteering torque T detected by the steering torque sensor 33 tends toincrease. However, if the assist current A is increased based on anincrease in the steering torque T, because the rack bar 20 is in thelateral limit position and the motor 29 cannot be rotated, there is apossibility that an excessive amount of current will flow through themotor 29 and cause overheating of the motor 29, or that an excessivepower will be output from the motor 29, thereby affecting the durabilityof the steering box 17.

The following is the detailed description of an overload preventingcontrol for the steering actuator 18 which is conducted when the assistcurrent A is equal to or larger than the second threshold value A1 orwhen the steering torque T is equal to or larger than the firstthreshold value T1.

In the overload preventing control, the assist current A in the motor 29of the steering actuator 18 is controlled so that equation (1) isestablished:

ΔT*+A=A1  (1)

wherein A1 is a second threshold value for the assist current A (seeFIG. 7), and ΔT* is a value of a current obtained by converting anexcess of the detected torque T over the first threshold value T1 (seeFIG. 7).

When equation (1) is modified with respect to the assist current A,equation (2) is obtained:

$\begin{matrix}\begin{matrix}{A = {{A\; 1} - {\Delta \; T^{*}}}} \\{= {{A\; 1} - \left( {T^{*} - {T\; 1^{*}}} \right)}} \\{= {\left( {{A\; 1} + {T\; 1^{*}}} \right) - T^{*}}} \\{= {\left( {{limit}\mspace{14mu} {value}} \right) - T^{*}}}\end{matrix} & (2)\end{matrix}$

wherein T* is a value of a current obtained by converting the steeringtorque T, and T1* is a value of a current obtained by converting thefirst threshold value T1 of the steering torque T.

Equation (2) produces a control that occurs where the sum of the assistcurrent A and the conversion current value T* of the steering torque Tis equal to a constant threshold value (=A1+T1*). In equation (2), whenthe conversion current value T* of the steering torque T is increased,the assist current A is decreased. Therefore, when the rack bar 20 hasreached the lateral limit position, it is possible to prevent anexcessive current from flowing through the motor 29 of the steeringactuator 18 and prevent an excessive load from acting on meshed portionsbetween the rack 19 and the pinion 21 of the steering box 17, therebypreventing the durability of the box 17 from being adversely affected.

For example, if A1=50 [A] (A1*=5,000 [kg·mm]) and T1=400 [kg·mm] (T1*=4[A]), when the rack bar 20 has reached the lateral limit position andT=1,000 [kg·mm] (T*=10 [A]) is input thereinto, equation (3) isestablished based on equation (2):

$\begin{matrix}\begin{matrix}{A = {\left( {{A\; 1} + {T\; 1^{*}}} \right) - T^{*}}} \\{= {\left( {50 + 4} \right) - 10}} \\{= {44\;\lbrack A\rbrack}}\end{matrix} & (3)\end{matrix}$

Thus, it is possible to decrease the assist current by 6 [A] from 50 [A]to 44 [A] to decrease the assist torque by 600 [kg·mm].

As is apparent from equation (1), the overload preventing controlmaintains ΔT*+A at the second threshold value A1 which is a fixed value.Therefore, even if the steering torque T is increased at the stroke end,the current supplied to the steering actuator 18 does not fluctuate,thereby providing a good steering feeling to the driver.

In the above-described overload preventing control, a value associatedwith a steering torque is converted into a current value, but even if avalue associated with the assist current is converted into a steeringtorque, the same effect is provided.

Specifically, the assist current A for the motor 29 of the steeringactuator 18 is controlled so that equation (1′) is established:

ΔA*+T=T1  (1′)

wherein T1 is the first threshold value (see FIG. 7) for the steeringtorque, and ΔA* is a value of the torque obtained by converting anexcess of the assist current A over the second threshold value T2 (seeFIG. 7).

When the equation (1′) is modified with respect to the steering torqueT, equation (2′) is obtained:

$\begin{matrix}\begin{matrix}{T = {{T\; 1} - {\Delta \; A^{*}}}} \\{= {{T\; 1} - \left( {A^{*} - {A\; 1^{*}}} \right)}} \\{= {\left( {{T\; 1} + {A\; 1^{*}}} \right) - A^{*}}} \\{= {\left( {{limit}\mspace{14mu} {value}} \right) - A^{*}}}\end{matrix} & \left( 2^{\prime} \right)\end{matrix}$

wherein A* is a value of the torque obtained by converting the assistcurrent A, and A1* is a value of the torque obtained by converting thesecond threshold value A1 for the assist current A.

The equation (2′) produces a control that occurs where the sum of thesteering torque T and the conversion torque value A* of the assistcurrent A is controlled so as to be equal to a constant limit value(=T1+A1*).

FIGS. 9 and 10 show a second embodiment of the present invention.

In the first embodiment, when the assist current A is equal to or largerthan the second threshold value A1 or the steering torque T is equal toor larger than the first threshold value T1, the overload suppressingcontrol is conducted on the presumption that the steering gear box 17has reached the stroke end. However, in the second embodiment, thestroke end is actually detected using a stroke end sensor unit 51.

As shown in FIG. 9, the stroke end sensor unit 51 includes adisplacement sensor 53 having a detecting element 53 a abutting aninclined cam face 20 a formed in a portion of a rack bar 20 where therack bar 20 is supported on a bearing 52. When the rack bar 20 islaterally moved, the detecting element 53 a abutting the cam face 20 amoves forward or rearward to change an output voltage v of thedisplacement sensor 53. When the output voltage v has become equal to orsmaller than v1 or equal to or larger than v2, it is detected that therack bar 20 has reached a left or right stroke end E1 or E2.

On a road surface having a small friction coefficient, such as an icyroad or a snow-covered road, a steering reaction force input from theroad surface to a tire is small. Hence, a steering torque detected bythe steering torque sensor 33 is small compared with a road surfacehaving an ordinary friction coefficient. Therefore, it is necessary todecrease the second threshold value A1 for the assist current A and thefirst threshold value T1 for the steering torque T in order to start theoverload preventing control. Thus, in the second embodiment, when thestroke end of the steering gear box 17 is detected by the stroke endsensor unit 51 to start the overload preventing control, if the assistcurrent A is smaller than the second threshold value A1 or if thesteering torque T is smaller than the first threshold value T1, thethreshold values A1 and T1 are decreased to threshold values A2 and T2,respectively (see FIG. 7).

As shown in FIG. 10, at Step S11, a steering torque T is detected by thesteering torque sensor 33, and a stroke end is detected by the strokeend sensor unit 51. At Step S12, an assist current A to be supplied tothe motor 29 of the steering actuator 18 is searched in an assistcurrent map 41 by using the steering torque T. When the stroke end ofthe steering gear box 17 is not detected by the stroke end sensor unit51 at Step S13, an electric power steering control is conducted at StepS14 similar to the control in the previously described first embodiment.

On the other hand, when the stroke end has been detected by the strokeend sensor unit 51 at Step S13, if the assist current A is not smallerthan the second threshold value A1 and the steering torque T is notsmaller than the first threshold value T1 at Step S15, the electricpower steering control is conducted at Step S17 based on the secondthreshold value A1 and the first threshold value T1. If the assistcurrent A is smaller than the second threshold value A1 or if thesteering torque T is smaller than the first threshold value T1 at StepS15, the threshold values A1 and T1 are decreased to threshold values A2and T2 (see FIG. 7) at Step S16 on the presumption that the road surfacehas a small friction coefficient, and the overload preventing control isconducted at Step S17 based on the decreased threshold values A2 and T2.

In this way, when the friction coefficient of the road surface is small,the second threshold value A1 for the assist current A and the firstthreshold value T1 for the steering torque T, which are used for theoverload preventing control, are decreased to A2 and T2, respectively.Therefore, even if the assist current A or the steering torque T issmall due to the small friction coefficient of the road surface when therack bar reaches the stroke end, the overload preventing control isreliably started. Further, the threshold value for the overloadpreventing control is decreased to a value smaller than the usual value,thereby more reliably preventing an overload.

Next, a third embodiment of the present invention will be described withreference to FIG. 11.

The stroke end sensor unit 51 in the second embodiment mechanicallydetects the position of the rack bar 20, but a stroke end sensor unit 51in the third embodiment detects a stroke end based on the rigidity ofthe steering gear box 17 relative to the transmission of the steeringtorque.

That is, a spring constant ktb of a torsion bar 34 of a steering torquesensor 33 and a spring constant kty of a tire are involved in a steeringtorque-transmitting system, and when the steering box 17 has reached astroke end, the spring constant kty of the tire is not substantiallyinvolved. Therefore, even if the assist current to be supplied to themotor 29 of the steering actuator 18 is increased, the output shaft 30of the motor 29 is difficult to rotate, wherein the stroke end isdetected.

Specifically, a rotational angle θ of the motor 29 is detected by aresolver, and if a value obtained by dividing the steering torque T bythe rotational angle θ is equal to or larger than a threshold value, itis determined that the rigidity of the steering gear box 17 is large,namely, the steering gear box 17 has reached the stroke end. If themotor 29 is a brushless motor, a rotation-controlling resolver mountedon the motor 29 is used. According to the third embodiment, thedisplacement sensor 53 in the second embodiment is not required, therebyreducing the overall cost of the invention.

Next, a fourth embodiment of the present invention will be describedwith reference to FIG. 12.

The above-described steering torque sensor 33 uses the torsion bar 34,but the fourth embodiment uses a magnetostriction-type steering torquesensor 33.

The magnetostriction-type steering torque sensor 33 includes first andsecond magnetostriction membranes 61A and 61B, which are made, forexample, of an Ni—Fe plating so as to cover a surface of a pinion shaft22 over a predetermined width; a first coil 38A surrounding the firstmagnetostriction membrane 61A; and a second coil 38B surrounding thesecond magnetostriction membrane 61B. A differential amplifying circuit39 is connected to the first coil 38A and the second coil 38B.

When a steering torque is input to the pinion shaft 22, an inductance ofthe first magnetostriction membrane 61A is changed from L to L+ΔL, aninductance of the second magnetostriction membrane 61B is changed from Lto L−ΔL, and the amount of change ΔL is proportional to the appliedsteering torque. Therefore, ΔL is detected by using the first and secondcoils 38A and 38B.

As shown in FIG. 6, the differential amplifying circuit 39 multiplies adifference between first and second output voltages VT1 and VT2 by again k to obtain a third output voltage VT3 (torque detection signal).The first output voltage VT1 increases with an increase in steeringtorque; the second output voltage VT2 decreases with the increase insteering torque; and hence, the third output voltage VT3 increases withthe increase in steering torque. When the steering torque is 0, thethird output voltage VT3 is biased to become a predetermined biasvoltage Vb (for example, 2.5 V).

VT3=k×(VT1−VT2)+Vb

When the third output voltage VT3 is calculated in this manner, apredetermined steering torque T is obtained from a map stored in acomputer (not shown) based on the third output voltage VT3 and a vehiclespeed.

The magnetostriction steering torque sensor of this embodiment does notuse the torsion bar 34, thereby increasing the rigidity of the steeringtorque transmitting system and also increasing the upper limit value onthe detectable steering torque. Particularly, when the stroke end sensorunit 51 in the third embodiment is employed, the increased rigidity ofthe steering torque transmitting system improves the accuracy ofdetection of the stroke end.

The embodiments of the present invention have been described above, butvarious changes in design may be made without departing from the subjectmatter of the present invention.

For example, the steering actuator 18 is mounted on the pinion shaft 22in the embodiments, but it may be mounted on the rack bar 20.

1. An electric power steering system comprising: a steering wheel; arack-and-pinion steering gear box connected to the steering wheel; asteering torque sensor for detecting a steering torque input to thesteering wheel; and a steering actuator driven by an assist currentcalculated based on the steering torque; wherein an overload preventingcontrol for limiting the driving of the steering actuator is conducted,wherein a sum of the assist current and a current conversion value ofthe steering torque detected by the steering torque sensor equals apredetermined threshold value.
 2. The electric power steering systemaccording to claim 1, wherein, when the steering torque detected by thesteering torque sensor is equal to or larger than a first thresholdvalue or when the assist current is equal to or larger than a secondthreshold value, the overload preventing control is conducted, andwherein the first and second threshold values are different from thepredetermined threshold value.
 3. The electric power steering systemaccording to claim 1, further including a stroke end sensor fordetecting a stroke end of the steering gear box, wherein when the strokeend is detected and the assist current is smaller than the predeterminedthreshold value, the overload preventing control is conducted using athreshold value that is less than the predetermined threshold value. 4.The electric power steering system according to claim 1, furtherincluding a stroke end sensor for detecting a stroke end of the steeringgear box, wherein when the stroke end is detected and the steeringtorque is smaller than the predetermined threshold value, the overloadpreventing control is conducted using a threshold value that is lessthan the predetermined threshold value.
 5. The electric power steeringsystem according to claim 3, wherein the stroke end sensor detects thestroke end based on rigidity of the steering gear box.
 6. The electricpower steering system according to claim 4, wherein the stroke endsensor detects the stroke end based on rigidity of the steering gearbox.
 7. The electric power steering system according to claim 1, whereinthe steering torque sensor is a magnetostriction torque sensor.
 8. Theelectric power steering system according to claim 2, wherein thesteering torque sensor is a magnetostriction torque sensor.
 9. Theelectric power steering system according to claim 3, wherein thesteering torque sensor is a magnetostriction torque sensor.
 10. Theelectric power steering system according to claim 4, wherein thesteering torque sensor is a magnetostriction torque sensor.
 11. Theelectric power steering system according to claim 5, wherein thesteering torque sensor is a magnetostriction torque sensor.
 12. Theelectric power steering system according to claim 6, wherein thesteering torque sensor is a magnetostriction torque sensor.